Method and apparatus for measuring luminosity with controlled sensitivity

ABSTRACT

Method for luminosity measuring advantageously in TLD evaluation equipment in the course of which a photomultiplier current is made proportional with light intensity and the current is measured; and sensitivity control of photomultiplier is conducted by varying its supply voltage if the current is above a given level. The change is stored. The measurement is conducted in analog or digital form. Circuit arrangement to implement the procedure has a photomultiplier tube (11), high voltage power supply and level sensor, the input of the photomultiplier tube being connected to the output of the high voltage power supply, the output of the tube being connected to the input of the level sensor. A control unit (14) is provided, whose input is connected to the output of the level sensor (12), and its output is connected to the input of the high voltage power supply. The level sensor may comprise analog to digital converter (21), digital to analog converter (22), comparator (23) and digital counter (24) connected in series to analog to digital converter (21), decimal counter (25) and control register (26) connected in series. The control unit (14) has a reference source (27) and calibrating divider (28), the inputs of which are connected to the output of the reference source (27). The calibrating divider has three resistors (R 1 , R 2 , R 3 ).

This application is a continuation of application Ser. No. 760,749,filed 5/22/85, now abandoned.

The object of the present invention is a method and a circuitarrangement for luminosity measuring, over a range of at least sixorders of magnitude, advantageously in "TLD" equipment.

In the field of luminosity measurements, photomultiplier tubes (PMT) arevery often applied in scientific and technical practice.

For their operation, such multiplier tubes require high voltage(500-1500 V) power suppllies. The sensitivity of the tubes dependsconsiderably on supply voltage, but in the case of a stable supplyvoltage the luminosity to current conversion ratio of the multipliertube - that is, its sensitivity - is also stable (if constanttemperature is assumed).

Other well-known devices are available for measuring luminosity, forexample vacuum photodiodes, solid state photodiodes andphototransistors, but their sensitivity is 3-4 orders of magnitude lowerthan the sensitivity of multiplier tubes. Therefore, to detect very lowlevel luminosities - for example 10⁻¹² -10⁻¹³ lumen intensity - onlymultiplier tubes are suitable and applied at present. Luminosity levelssuch as this must be processed in scientific and technical practice,e.g. in the field of thermoluminescent dosimetry (TLD) or measuringsignals with scintillation detectors.

But in these fields, not only do too low luminosity levels occur, butlevels 8 orders of magnitude higher, say for example 10⁻⁴ -10⁻⁵ lumenmagnitude, also occur. If a multiplier tube is set to sense the lowerluminosity levels - which means greater sensitivity - it may becomesaturated in the case of too great a light intensity, that is, it "goesblind".

Moreover, because of the large output current, we have to apply a highcurrent resistor-network, which means a needlessly high load for thehigh voltage power supply in most cases, and this heats the multipliertube thereby reducing its stability.

The current output by the multiplier tube is usually displayed indigital form by means of analog to digital converters, which may beluminosity level sensing, but can also operate by summing the luminosityover a certain time interval, such as the method developed inthermoluminescent does measurements.

In the case of a digital display, generally 3-4 digits of readout arerequired because of the necessary 0.1% measuring accuracy. For a 3-4digit display, we need to use a decimal point and display the exponent,in order to cover a measuring range of 6-8 orders. In view of this, theoperation of the signal processing circuit must be appropriatelymodified. A convenient way of doing this is to change the conversionfactor of the analog to digital converter; but this method does notprevent saturation of the multiplier tube if higher luminosity levelsare applied.

The present invention seeks to solve the problem in "TLD" measurementsby eliminating the saturation of multiplier tubes in the case of greaterlight intensities on the one hand, and by making possible the measuringof luminosity over more orders of magnitude on the other.

Thus the task to be solved by this invention may be considered to be thecreation of a method and circuit arrangement to reduce the saturation orsensitivity of the multiplier tube by a well-defined factor, thusrendering it suitable for measuring light quantities over more orders ofmagnitude.

The invention is based on the concept that the sensitivity of themultiplier tube may be very precisely set by changing the input voltage,and it can be changed over more orders of magnitude, and in the case ofswitching from a higher voltage or sensitivity to a lower one, themultiplier tube can rapidly adjust to the lower sensitivity level. Forexample in thermolluminescent dosimetry the time required for lightquantity measurements is about 10-30 seconds. The time required forswitching is 10-20 milliseconds. If the worst case is presumed, theerror caused by this is less than 0.1%. For example this error isinvisible in the case of three-digit display and on the other hand theprobable accuracy of "TL" dosimetry is not better than 0.5% so an errorof 0.1% is negligible.

The method applied in this invention is an improvement of anotherwell-known procedure in which a current is produced that is proportionalto the light intensity, and this current is measured, and the luminosityto current conversion is performed by a photomultiplier.

The improvement according to the invention is that at the same time orin parallel with current measuring we compare the current, and if thecurrent is higher than a given level, we decrease the sensitivity of thephotomultiplier, advantageously by one order of magnitude, by changingthe supply voltage, and this change in order is stored, for example, bya register, and measurement and comparison are then continued.

In the sense of the invention it is practical to compare the current tobe measured using a linear circuit. In other words, it is practical totransform the current to a digital signal, and then to measure andcompare this digital signal.

The circuit arrangement applied in this invention is an improvement of awell-known circuit arrangement which contains a photomultiplier tubewith a high voltage power supply and level sensor, the input of thephotomultiplier tube being connected to the output of the high voltagepower supply, and the output of the tube being connected to the input ofthe level sensor.

The improvement according to the invention is that this circuitarrangement also has a control unit whose input is connected to theoutput of the level sensor through a control line, and whose output isconnected to the input of the high voltage power supply through areference line.

In the sense of the invention it is practical if the level sensor has ananalog to digital converter, a digital to analog converter, a comparatorand a digital counter, connected in series in such a way that the countinput of the digital counter is connected to the output of the analog todigital converter through an impulse line, and the order-select input ofthis counter is connected to the output of the comparator through thecontrol line.

Namely it is practical if the level sensor has an analog to digitalconverter, decimal counter and control register, each connected inseries in a way that the control register output is connected to thepreset input of the decimal counter through the preset line.

Moreover it is practical if the control unit has a reference source anda calibrating divider, the inputs of the said calibrating divider beingconnected to the output of the reference source through a stable voltageline and also to the control line. The output of the calibrating divideris connected to the reference line.

Moreover it is also practical, if the calibrating divider has threeresistors designated hereafter as first, second and third, each resistorbeing connected to the reference line by one pole, the other pole of thefirst resistor being connected to the stable voltage line, and the otherpole of the second resistor being connected to ground and the other poleof the third resistor being connected to the control line.

The invention is presented in more detail hereinafter with reference tothe accompanying drawings, in which:

FIG. 1 illustrates the timing of the method applied in the invention;

FIG. 2 illustrates an embodiment of the circuit arrangement applied inthe invention;

FIG. 3 shows a possible realization of a level sensor applied in theinvention;

FIG. 4 shows another possible realization of level sensor, and

FIG. 5 shows a possible configuration of the control units applied inthe invention.

In the diagram of FIG. 1 we introduce the summing method ofthermoluminescent dosimetry based on the principle of sensitivitymodification. If a thermoluminescent dosimeter is evenly heated it willradiate light after a certain time. The increase of temperature or time,on the abscissa, causes an increase in the radiated light, on theordinate, and later this light decreases with increasing temperature. Ifa light-intensity diagram is plotted it shows a bell-shaped curve whichis known as the glow curve. The area below this curve or the total lightquantity is proportional to the dose and the purpose of TLD measurementis to determine this area. The g_(max) light-current is the saturationcurrent of the multiplier tube and in the normal case the tube cannotfollow the "glow" curve so the supply voltage of the multiplier tube isdecreased with the state procedure. As an example, the sensitivity ofthe multiplier tube is decreased by one order of magnitude, and then wecontinue the measuring of light quantity and the order-change isdisplayed on the decimal display. The total light summing time T_(x) isgenerally 10-30 seconds, and the switching time T₁ is les than 10milliseconds so the uncertainty of the summing caused by the switchingtime is negligible, as may be seen in the figure.

In FIG. 2 one possible circuit arrangement is shown according to theinvention. This circuit arrangement has a photomultiplier tube 11, highvoltage power supply 13 and level sensor 12. The input of thephotomultiplier tube 11 is connected to the output of the high voltagepower supply 13 through the voltage line f, and the output of the tubeis connected to the input of the level sensor 12 through the currentline m. The circuit arrangement also has a control unit 14. The input ofthe control unit 14 is connected to the output of the level sensor 12through the control line v, and the output of the control unit isconnected to the input of the high voltage power supply 13 through thereference line r.

The high voltage power supply 13 feeds the multiplier tube 11 which inturn senses light, for example, that emitted by the TLD, and thelight-current of the multiplier tube 11 reaches the level sensor 12through the current line m. At a certain current level, in practiceslightly lower than the saturation current g_(max) of the multipliertube 11, the level sensor gives a switching signal to the control unit14 through the control line v and control unit 14 then determines theoutput voltage of the high voltage power supply 13 through referenceline r. The effect of the switching signal causes the control unit 14 todecrease the output voltage of the high voltage power supply 13 throughthe reference line r. It is practical to set the decrease to be roundedoff to the next order of magnitude lower than the original value.

In FIG. 3 a possible realization of the level sensor according to theinvention is shown. The level sensor 12 has an analog to digitalconverter 21, a digital to analog converter 22, a comparator 23 and adigital counter 24 all of which are connected in series. The count inputof the digital counter 24 is connected to the output of the analog todigital converter 21 through the impulse line i, the order-select inputof the counter is connected to the output of the comparator 23 throughcontrol line v.

The current of the multiplier tube 11 is converted to impulses by theanalog to digital converter 21, and the frequency of these impulses isdirectly proportional to the current of the multiplier tube. Theimpulses reach the digital to analog converter 22 and the digitalcounter 24 count input through the impulse line i. The digital to analogconverter 22 transmits a voltage to the analog line d that isproportional to the frequency, and this line is connected to the inputof the comparator unit 23. The comparator unit 23 is set so that itsthreshold is slightly lower than the saturation current of themultiplier tube. At this said level it gives a switching signal to thecontrol unit 14 through the control line v, as well as to theorder-select input of the digital counter 24.

In FIG. 4 another example of a circuit arrangement for a level sensoraccording to the invention is shown. According to this the level sensor12 has an analog to digital converter 21, decimal counter 25 and controlregister 26, connected in series. The output of the control register 26is connected to the preset input of the decimal counter 25 through thepreset line b. The impulses of the analog to digital converter 21 reachthe decimal counter 25 through the impulse line i. The overflow outputof the decimal counter 25 transmits a carry impulse to the input of thecontrol register 26 through the carry line c. The control register 26transmits a switching signal to the order-select input of the decimalcounter 25 through the preset line b, as well as switching the controlunit 14 through the control line v.

In FIG. 5 a possible realization of the control unit according to theinvention is shown. Thus, the control unit 14 has a reference source 27,and a calibrating divider 28. The inputs of the calibrating divider 28are connected to the reference source 27 through the stable voltage lines and to the control line v, its output being connected to the referenceline r.

The calibrating divider according to the invention is also shown in FIG.5. The calibrating divider 28 has three resistors R1, R2, R3, eachconnected to the reference line r by one pole. The other pole of thefirst resistor R1 is connected to the stable voltage line s; the otherpole of the second resistor R2 is connected to ground; and the otherpole of the third resistor R3 is connected to the control line v.

The most usual treatment of the high voltgae power supply 13 is that itsoutput voltage is compared with a reference voltage by an internalcircuit. The output voltage is in a closed connection with the referencevoltage which appears on the reference line r in the drawings. In thisrealization the voltage of the reference source 27 is divided to thedesired value by the calibrating divider 28. The calibration occurs suchthat the voltage divided by the R1 and R2 resistors adjusts the highvoltage power supply 13 to the original value through the reference liner. The level obtained through the control line v, for example, 0 V,switches the R3 resistor into the division. The voltage of the referenceline r is thereby varied and so the high voltage is also changed. It ispractical to adjust the value of resistors R1, R2, R3 so that the variedhigh voltage would make the sensitivity of the multiplier tube 11decrease by entire orders of magnitude.

The advantages of the method and circuit arrangement according to theinvention may be summarized as follows:

it prevents the saturation of the multiplier tube thereby eliminatingthe non-linearity error of luminosity to current conversion.

the measuring range of the photosensitive system is increased by moreorders of magnitude

the divider circuit of the multiplier tube may be of a low current typewhereby the load and the noise of the high voltage power supply aresmaller

the electrical power dissipation is lower, which means less heating ofthe tube and thus the temperature of the tube decreases and thestability of the tube increases.

We claim:
 1. In a method of thermoluminescent dosimetry comprising thesteps of:heating a thermoluminescent material to generate a luminousflux; using a photomultiplier supplied with an input voltage to detectluminosity corresponding to said generated luminous flux and to generatea current proportional to said detected luminosity; and measuring saidgenerated current over a time interval corresponding to said generatedluminous flux; wherein said photomultiplier has a saturation thresholdabove which said luminosity corresponding to said generated luminousflux is not detected; the improvement comprising: decreasing thesensitivity of said photomultiplier by decreasing its said input voltageeach time said generated current attains a predetermined valuecorresponding to a said luminosity at or below said saturationthreshold, each said decrease in input voltage being irreversible duringsaid time interval.
 2. In a thermoluminescent dosimeter comprising:athermoluminescent material; a photomultiplier supplied with an inputvoltage, detecting luminosity corresponding to luminous flux generatedby said thermoluminescent material and generating a current proportionalto said detected luminosity, said photomultiplier having a saturationthreshold above which said luminosity corresponding to said generatedluminous flux is not detected; and means measuring said generatedcurrent over a time interval corresponding to said generated luminousflux; the improvement comprising: means comparing said measured currentto a predetermined value corresponding to a said luminosity at or belowsaid saturation threshold; and means decreasing said input voltage eachtime said comparing means detects said measured current has attainedsaid predetermined value, each said decrease in input voltage beingirreversible during said time interval.